The present invention relates generally to an information communication system and, particularly, to an information communication system including a signal selecting apparatus for effectively servicing subscribers with desired pieces of information, such as image signals and multichannel audio signals, in response to the request of the subscribers.
Recently, attention is being paid to new communication systems for supplying various desired information to the homes of subscribers. For example, a terminal unit provided in the home is operated by the user to specify a desired combination of image and audio signals such as stereo sound. Various systems have been considered to realize this kind of system. One example of such a system has selector units for selecting the information signals to be sent to the subscriber located in a relay station known as a sub center. Such systems using sub centers are desirable, since unauthorized reception can be readily prevented, maintenance is easy and the leving of fees simple.
Information signals supplied from the sub center to the subscriber include various different kinds such as (1) signals supplied from the center to the sub center, (2) signals transmitted from a geostationary satellite to the sub center, and (3) signals supplied independently by the sub center, for example, package system media such as video discs. The largest proportion of the signals supplied to the subscriber are from a center to the sub center. With a system in which image signals, multichannel sound signals (stereo signals included with the image signals, or separate audio signals) and digital signals, such as character data, are sent from the center to the sub center, optical fibers are most suitable. In order to improve the transmission efficiency of the different signals over the optical fiber, a frequency-division multiplexing scheme is used.
Suitable signals in the frequency-division multiplexing scheme are selected via an analog matrix switch in response to the demands of the subscribers. With optical fiber transmission, image signals are sent in a baseband of several megahertz or less, while sound and data signals are sent spread over a higher frequency band. However, with this kind of simple frequency-division multiplexing system, at least four kinds of signals in the entire communication frequency band (one image signal, two sound signals and one data signal) are multiplexed, so there is a problem of image degradation due to different frequency signal components between neighbouring signal on the frequency axis leaking into the image signal. This mixing of different frequency signals is the cause of nonlinear distortion being generated at the wide band analog matrix switch and/or the electrical/optical converting section. This problem becomes more serious with the increase in the number of signals and with the increase in power level of the sound and data signals. In other words, with the prior art frequency-division multiplexing system it is not possible to effectively transmit high quality information. If the power level is increased in order to prevent the different frequency components from being mixed with the image signal, crosstalk from the image signal to the sound signal increases, resulting in a buzzing sound. A frequency band width of 15 kHz is not sufficient to prevent this kind of crosstalk.